PNNL team develops high-capacity nanocomposite anode for Na-ion batteries based on multi-component alloying reactions

13 February 2012

Rate capability of the SnSb/C nanocomposite electrode at various current rates from 100 to 1000 mAh g-1. Xiao et al. Click to enlarge.

Researchers at US Department of Energy (DOE) Pacific Northwest National Laboratory have demonstrated a new tin-antimony (SnSb/C) nanocomposite based on sodium (Na) alloying reactions as an anode for Na-ion battery applications. The electrode achieves an exceptionally high capacity of 544 mAh g^-1 (almost doubling that of intercalation carbon materials), good rate capacity and cyclability (80% capacity retention over 50 cycles) for Na-ion storage.

In 2011, the team reported on single crystalline sodium-manganese oxide (Na₄Mn₉O₁₈) nanowires for sodium-ion battery applications that showed a reversible capacity of 128 mAh g^-1 and stable cycling for more than 1,000 cycles in a paper in the journal Advanced Materials. (Earlier post.) A paper on the new nanocomposite has been accepted for publication in the RSC journal Chemical Communications.

Sodium has been proposed as a promising lower-cost alternative to Li-ion rechargeable batteries for grid storage. However, sodium-sulfur batteries currently in use run at temperatures above 300 °C, making them less energy efficient and safe than batteries that run at ambient temperatures. Sodium-ion batteries have been discussed in the literature for some time, and are attractive because they could potentially be less expensive, safer, and more environmentally benign. However, Na ions are significantly larger in radius than Li ions, making it difficult to find a suitable host material to accommodate the Na ions and allow reversible and rapid ion insertion and extraction.

Li alloys have been extensively investigated as high capacity anodes for Li-ion batteries. Analogously, Na can alloy with many metallic elements such as Sn, Sb, germanium (Ge), lead (Pb), and the calculated theoretical specific capacities are 847 (Na₁₅Sn₄), 660 (Na₃Sb), 1,108 (Na₃Ge), and 484 (Na₁₅Pb₄) mAh g^-1 respectively. Na alloying reactions also have slightly higher thermodynamic potentials than that of Li, making them potentially safer. Still, to date, there have been very limited experimental studies of alloy compounds for reversible Na ion insertion. Here, we provide the first report of high capacity alloy reaction for Na ion insertion based on SnSb/C nanocomposites with a reversible capacity over 500 mAh g^-1.

—Xiao et al.

For the study, they prepared SnSb/C nanocomposites with a metal:carbon black (Super P) weight ratio of 7:3. The Sn:Sb ratio was 1:1 by molar ratio. Among the findings of their study:

• Initial discharge gives an overall capacity of 742 mAh g^-1. The first charge recovers a reversible capacity of 544 mAh g⁻¹, with an initial coulombic efficiency of 75.1%, which quickly improves after a few cycles.

• For the metal/C composites, some capacity is derived from the carbon matrix. Pure ball-milled super P can deliver a stable specific capacity of 172 mAh g^-1. With 30 wt.% Super P in the composite, the carbon may contribute to about 51.6 mAh g^-1 in capacity. Therefore, most of the reversible capacity (about 492.4 mAh g^-1) comes from the alloy.

• For a 50% SnSb alloy, the theoretical capacity is assumed to be 753 mAh g^-1 based on 50% Na₁₅Sn₄ (847 mAh g^-1) plus 50% Na₃Sb (660 mAh g^-1). For 70 wt.% active alloy, the theoretical capacity is then 527 mAh g^-1. Therefore, the reversible capacity from the active alloy material is about 93.4% of the theoretical capacity—almost two times higher than best results obtained from carbon-based anode materials.

• After 50 cycles, the electrode maintains a capacity of 435 mAh g^-1. The coulombic efficiency of the electrode is as high as 98.3% for more than 50 cycles, indicating good reaction reversibility.

• The electrode delivers a discharge capacity of 433, 337, and 274 mAh g^-1 at high charge/discharge rates of 200, 500, and 1000 mA g^-1, respectively. When the rate is reset to 100mA g^-1 after 50 cycles, the capacity recovers to 419 mAh g^-1 and retains 81% of the original capacity, which indicates good tolerance for the rapid Na ion insertion/extraction.

Based on the CV results and charge-discharge behavior, we suggest that the coexisting Sn- and Sb-rich phases likely formed during sequential electrochemical reactions may self-support one another. When one of the phases is deformed, the other phase may retain the stability and maintains good electric contact during the alloying- dealloying processes. The multicomponent alloy reaction represents an important direction for developing high-capacity electrode materials for Na-ion batteries. Optimization of the alloy composition and structure and careful study of the structural change of the different alloy phases during the reactions will lead to further improvement of the electrochemical properties.

—Xiao et al.

Resources

• Lifen Xiao, Yuliang Cao, Jie Xiao, Wei Wang, Libor Kovarik, Zimin Nie and Jun Liu (2012) High capacity, reversible alloying reactions in SnSb/C nanocomposites for Na-ion battery applications. Chem. Commun., 2012, Accepted Manuscript doi: 10.1039/C2CC17129E